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Exp Brain Res (2008) 184:283–293
DOI 10.1007/s00221-007-1107-9
123
RESEARCH ARTICLE
The coordination patterns observed when two hands
reach-to-grasp separate objects
GeoVrey P. Bingham · Kirstie Hughes ·
Mark Mon-Williams
Received: 23 May 2007 / Accepted: 23 July 2007 / Published online: 29 August 2007
© Springer-Verlag 2007
Abstract What determines coordination patterns when
both hands reach to grasp separate objects at the same time?
It is known that synchronous timing is preferred as the most
stable mode of bimanual coordination. Nonetheless, normal
unimanual prehension behaviour predicts asynchrony when
the two hands reach towards unequal targets, with syn-
chrony restricted to targets equal in size and distance. Addi-
tionally, suYciently separated targets require sequential
looking. Does synchrony occur in all cases because it is
preferred in bimanual coordination or does asynchrony
occur because of unimanual task constraints and the need
for sequential looking? We investigated coordinative tim-
ing when participants (n = 8) moved their right (preferred)
hand to the same object at a Wxed distance but the left hand
to objects of diVerent width (3, 5, and 7 cm) and grip sur-
face size (1, 2, and 3 cm) placed at diVerent distances (20,
30, and 40 cm) over 270 randomised trials. The hand move-
ments consisted of two components: (1) an initial compo-
nent (IC) during which the hand reached towards the target
while forming an appropriate grip aperture, stopping at (but
not touching) the object; (2) a completion component (CC)
during which the Wnger and thumb closed on the target. The
two limbs started the IC together but did not interact until
the deceleration phase when evidence of synchronisation
began to appear. Nonetheless, asynchronous timing was
present at the end of the IC and preserved through the CC
even with equidistant targets. Thus, there was synchrony
but requirements for visual information ultimately yielded
asynchronous coordinative timing.
Keywords Bimanual · Prehension · Movement ·
Coordination · Attention
Introduction
It is a matter of common observation that adult humans are
skilled at coordinating the right and left hands when reach-
ing to grasp two separate objects at the same time. The
question is whether the two hands are synchronised in time
during such reaches? There has been little investigation into
how the nervous system coordinates the actions of the two
limbs in bimanual prehension. Those studies that explored
bimanual prehension tended to investigate the two hands
reaching to a single object (e.g. Castiello et al. 1993; Tresil-
ian and Stelmach 1997). Jackson et al. (1999) conducted
one of the few studies into bimanual prehension directed at
two separated objects. Jackson et al. (1999) found that
bimanual movements took longer but were otherwise
unaVected as to whether the actions required of each hand
were the same or diVerent. Jackson et al. suggested that
synchronising the limbs to a Wxed duration solves the prob-
lem of executing incongruent bimanual movements. Jack-
son et al.’s study has made an important contribution to our
understanding of bimanual prehension but it leaves some
issues unresolved. In particular, it remains unclear as to
how the nervous system produces coordinated movements
to targets of diVerent sizes at diVerent distances.
The results of previous bimanual coordination studies
involving tasks other than prehension suggest that the
G. P. Bingham
Department of Psychology, Indiana University,
Bloomington, IN 47405, USA
e-mail: gbingham@indiana.edu
K. Hughes · M. Mon-Williams (&)
College of Medicine and Life Sciences,
University of Aberdeen,
Aberdeen AB24 2UB, Scotland, UK
e-mail: mon@abdn.ac.uk
284 Exp Brain Res (2008) 184:283–293
123
hands often interact and show synchronous coordination,
that is, each hand doing a similar thing at the same time
(e.g. for rhythmic movement see Kelso et al. 1979a,b; for
discrete movements see Diedrichsen et al. 2001). The most
widely studied bimanual coordination tasks involve rhyth-
mic movement of diVerent limbs, for instance, the left and
right wrists. Such movements interact to produce character-
istic stable modes of coordination with synchronous move-
ment being the most stable (see, for instance, Kelso 1995).
The coupling between the two movements is often based on
visual information as veriWed in studies where two diVerent
people each move a joint rhythmically and coordinate
movement (e.g. Schmidt et al. 1990). Subsequently, a large
number of studies have established the role of perception in
coupling the limbs in bimanual coordination (Bingham
2001, 2004 a, b; Bingham et al. 1999; Bingham et al. 2000;
Liao and Jagacinski 2000; Mechsner et al. 2001; Schmidt
et al. 1990; Serrien et al. 2001; Wilson and Bingham 2005
a, b; Wilson et al. 2003; Wimmers et al. 1992). Indeed,
Bingham (2001, 2004a,b) has attributed the characteristic
stable modes of bimanual rhythmic movement to percep-
tual information variables hypothesized to couple such
movements (see also Wilson and Bingham 2005 a, b).
The study of rhythmic movement is informative but it is
also important to study how people move the two limbs
together when carrying out a wide range of everyday ‘dis-
crete’ bimanual tasks. Kelso et al. (1979 a, b) and Diedrich-
sen et al. (2001) both studied bimanual reaching (‘aiming’)
movements and found that the left and right limbs inter-
acted and showed synchronous timing. We wished to inves-
tigate as to whether the two limbs interact in the same way
when bimanual reach-to-grasp movements are made to two
spatially separate target objects. The work of Kelso (1979
a, b) and others led us to expect that the two hands would
start moving together (and end in reasonable temporal
proximity) but we predicted asynchronous coordinative
timing for two reasons. First, the timing of uni-manual
reach-to-grasp movements varies reliably with the distance
of the target as well as with its grasping surface area (i.e.,
reaches to targets that are more distant or smaller require a
longer time to complete, e.g. Jeannerod 1984, 1988).
Bimanual reaches to targets diVerent in size and/or distance
should thus be expected to yield asynchronous timing. The
second reason we expected asynchronous timing is because
precision prehension requires vision to guide reaches but it
is not possible to Wxate two objects simultaneously when
objects are suYciently separated. Asynchronous coordina-
tion would be expected to allow time to look from one
object to the other while guiding the hands. We will con-
sider these two reasons in some depth in the subsequent
paragraphs.
Prehension exhibits regularities that should be expected
to introduce asymmetries in the context of bimanual coordi-
nation. The pattern of behaviour observed in unimanual
prehension experiments is highly stereotypical across and
within participants (Haggard and Wing 1995; Paulignan
et al. 1991; Jackson et al. 1997). If a precision grip (Napier
1956) is used, as the hand starts forward, the thumb and
Wnger move apart to form a maximum grip aperture that is
scaled relative to the size of the target object (e.g. Jeann-
erod 1984, 1988; Smeets and Brenner 1999). The hand
accelerates to a peak speed and then decelerates as the hand
approaches the target. The movement of the hand exhibits a
smooth ‘bell-shaped’ speed proWle with a relatively longer
deceleration phase. The maximum grip aperture occurs dur-
ing the deceleration phase when closing of the aperture is
initiated (Jeannerod 1984, 1988; Smeets and Brenner 1999;
Mon-Williams and Tresilian 2001). The reach movement
ends at the object with the thumb and Wnger closing to
Wnally contact and acquire the target object (Wing and Fra-
ser 1983). A larger target distance yields a larger peak
speed, but not large enough to prevent longer movement
durations (i.e. movement duration increases with reach dis-
tance). Increasing the accuracy demands of the task by
decreasing the grip surface size also produces longer dura-
tion movements. A smaller sized target at a given distance
will yield a slower movement (e.g. Bootsma et al. 1994;
Tresilian and Stelmach 1997; Tresilian et al. 1997;
Mon-Williams and Tresilian 2001). Likewise, the grasp
component shows stereotypical patterns of behaviour. The
maximum grip aperture is sensitive to the size of the object
so that the aperture is a little wider than the width of the
target object and occurs later in the movement for larger
objects (Jeannerod 1984, 1988; Smeets and Brenner 1999;
Mon-Williams and Tresilian 2001). It can be seen that
synchronous prehensile coordination to targets of diVerent
sizes and at diVerent distances would entail changes in the
regular patterns of prehension.
Nevertheless, even if bimanual prehension were per-
formed to separate targets of equal distance and size, we
would expect to Wnd evidence of asynchronous behaviour.
The reason is that targeted prehension requires visual guid-
ance for accurate performance (Fisk and Goodale 1988).
This is why prehension movements to targets with a smaller
grip area have a longer duration. That is, prehension move-
ments require online correction of errors under visual guid-
ance and thus exhibit the speed-accuracy trade-oV typical
of visually guided actions (e.g. Bootsma et al. 1994; Fitts
1954; Wing et al. 1986). In bimanual reaching-to-grasp, the
two targets cannot be Wxated at the same time when they
are spatially separate (see Riek et al. 2003). Accurate per-
formance under these conditions requires a sequential orga-
nisation that allows the two targets to be Wxated in turn to
guide the corresponding prehension. Indeed, Riek et al.
(2003) found ‘hovering’ behaviour in bimanual targeted
reaching (aiming) movements where one hand would hover
Exp Brain Res (2008) 184:283–293 285
123
and wait while the other hand was targeted (in that study,
targets were contacted by a probe and not grasped). It fol-
lows that any noise in the execution of the organised
sequence will result in asynchronous coordination—and
noise is an intrinsic feature of such actions (hence the need
for visually guided corrections in the Wrst place).
In the current study, we investigated the organisation
and timing of bimanual reaching-to-grasp. First, we were
interested in investigating reaches to targets at identical dis-
tances. This condition was predicted to yield the same
arrival times for the two hands. Nevertheless, we expected
to see asynchrony reXecting the demands of visually guid-
ing two hands in the Wnal stages of the movements. All else
being equal, we could not predict which hand would close
Wrst in asynchronous trials. We also manipulated the diY-
culty of the task. With greater diYculty, we expected to see
greater asynchrony. We varied the diYculty of targeting in
two ways. We varied the grip area available for contacting
the object on its sides with Wnger and thumb (1, 2, and
3 cm). Smaller grip area is more diYcult (in Fitts’ 1954
sense of requiring more visually guided corrections). We
also varied the frontoparallel width of the targets (3, 5, and
7 cm). Wider targets present a larger collision hazard (Meu-
lenbroek et al. 2001; Rosenbaum et al. 1999) and thus are
more diYcult. With two targets of unequal diYculty, it was
unclear as to whether one should predict closure on the
more or less diYcult target Wrst. So, while we looked for
ordering eVects, we only predicted increasing asynchrony.
Second, we investigated reaches to targets at diVerent
distances. This condition was predicted to yield diVerent
arrival times for the two hands. Nevertheless, should we
expect to see synchronous arrival reXecting the tendency in
bimanual coordination for synchrony? We manipulated the
target distance for the left hand (20, 30, and 40 cm) while
the distance of the target for the right hand remained con-
stant at the middle distance (30 cm). When the variable tar-
get was either closer or farther than the constant target then
independent reaching behaviour should naturally introduce
asynchrony in arrival times. The question was whether this
asynchrony would occur, and then be maintained during
closure. If so, then the order in which grasping was
achieved between the hands should vary as a function of the
relative target distances (i.e. the nearer target should be
grasped Wrst). We also manipulated the diYculty of grasp-
ing at diVerent distances so that distance would interact
with grip area and object width to determine diYculty.
Once more, we expected greater asynchrony when the diY-
culty of grasping was greater.
The design of the study was such that one hand (called
the “constant hand”) always reached to the same target
object. The other hand (called the “variable hand”) reached
to targets that varied in width, grip surface area and dis-
tance. We studied timing measures of the constant hand in
terms of the levels of object width, grip surface area, and
distance encountered by the variable hand. Variations in the
timing of the constant hand would indicate that its behav-
iour was aVected by the action of the variable hand (i.e. that
there were interactions between the hands). The question
then would be as to whether the interactions (1) would yield
synchronous timing when targets were at diVerent distances
or (2) would yield asynchronous timing when the targets
were at the same distances?
Methods
Eight unpaid participants from the University of Aberdeen
were recruited for the study (Wve females and three males
aged between 20 and 30 years, mean age 24 years). An
additional ten unpaid volunteers participated in a control
experiment (eight females and two males aged between 21
and 26 years). All participants had normal or corrected to
normal vision and none had any history of neurological
deWcit. The participants all reported a right hand preference
and all wrote and threw a ball with their right hand. All par-
ticipants provided their informed consent prior to their
inclusion in the study. The study was approved by a univer-
sity ethics committee and was performed in accordance
with the ethical standards laid down in the Declaration of
Helsinki.
The experimental task required the participants to sit at a
table and reach for two objects (dimensions of objects:
length 9.5 cm; height 3.2 cm; width: varying sizes). Two
separate starting positions were located 10 cm from the
edge of the table closest to the participant. The starting
positions were 45 cm apart equally spaced to the left and
right of the participants’ midline. Participants started each
trial with the left hand on the left-side starting position and
the right hand on the right-side starting position. In the con-
stant (right) hand workspace an object was placed along the
sagittal plane at 30 cm from the starting position. The
block, with a width of 5 cm and with a grip surface of 2 cm
diameter, remained in this position for the entire session
(Fig. 1). In the variable (left) hand workspace, the target
object varied in distance, width and grip surface area. Three
object amplitudes were used: 20, 30, or 40 cm from the
starting position along the sagittal plane. Nine objects were
used. They were composed by crossing three object widths:
3, 5, and 7 cm and three grip surface sizes: 1, 2 or 3 cm
diameter. Each target consisted of a block of wood with a
wooden dowel Wxed in it so that the ends of the dowel
extended to either side of the block by 2 cm (like buttons).
See Fig. 1. The dowel was to be held pinched between
Wnger and thumb each in contact with an end of the dowel.
The block held the dowel suspended about 2 cm above a
table surface. We varied the width of target objects by altering
286 Exp Brain Res (2008) 184:283–293
123
the width of the block in which the dowel was inserted.
Altering the diameter of the dowel varied the grip surface
area. To be clear, the width determined the distance by
which the tips of the index Wnger and thumb were separated
whilst holding the object in the grip. The grip surface area
was the size of each of the surfaces contacted by the thumb
and Wngertip, respectively.
Participants performed ten test trials in each of the 27
diVerent conditions (9 objects £ 3 distances) but Wrst par-
ticipants performed ten practice trials in which the left and
right amplitudes and objects were the same. Following this,
the 270 test trials were completed with the trial order rando-
mised across and between the participants. The entire ses-
sion, including practice trials, lasted »1 h. Participants
were informed that they should grasp the objects as quickly
and accurately as possible between the pads of the foreWn-
ger and thumb and that they should not lift the object oV the
table.
In the control condition, the variable hand reached to all
three distances but only the medium width block was used
and only the narrow and wide grasp surface. This gave a
total of six conditions with participants reaching eight times
(randomised order) in each condition (total = 48 trials). In
the control condition, two IREDs were attached securely to
a head-mounted frame that allowed us to track head posi-
tion. The signal was calibrated by asking participants to
alter Wxation between points on the tabletop by moving
their head. We decided to use head movement to monitor
Wxation as previous research has shown that gaze shifts are
implemented by changes in head position when angular
separation exceeds 20° (Stahl 1999) as was the case in the
current experimental setup.
The acquisition of data was initiated approximately one
second before the experimenter’s verbal start command.
The objects were visible to the participants when the “go”
signal was given. Infra red emitting diodes (IREDs) were
attached to the index Wnger (distal medial corner of the
Wnger) thumb (distal lateral corner of the thumb) and
the styloid process of the wrist for each reaching hand. The
positions of the IREDs were recorded by an Optotrak
movement recording system, factory precalibrated to a
static positional resolution of better than 0.2 mm at 250 Hz.
The data were collected for 3 s at 100 Hz and stored for
subsequent oVline analysis and Wltered using a dual-pass
Butterworth second-order Wlter with a cut-oV frequency of
16 Hz (equivalent to a fourth-order zero phase lag Wlter of
10 Hz). The distance between the thumb and index Wnger
IREDs was then computed (the aperture). Following this
operation, the tangential speed of the wrist IRED and the
aperture was computed and the onset and oVset of the
movement were estimated using a standard algorithm
(threshold for movement onset and oVset was 5 cm/s). Cus-
tom analysis routines were used to compute the dependent
variables of interest in this study.
The majority of hand movements were found to consist
naturally of two components: (1) an initial component (IC)
during which the wrist reached towards the target while
forming an appropriate grip aperture, stopping with the
Wngers surrounding (but not touching) the target object; (2)
a completion component (CC) during which the Wnger and
thumb closed on the target. That is, when the wrist had
stopped (ending the reach), the grasp remained to be com-
pleted, that is, the Wngers were not yet in contact with the
target object. A Wnal movement of the Wngers closed the
grasp with W
ngers in contact with the target object surfaces.
The two components were easily identiWed when the kine-
matic proWles were plotted for each trial (see Fig. 2). We
quantiWed these diVerent components and reported the sta-
tistics conducted on these data.
The criterion for onset of a reach was wrist speed exceed-
ing 5 cm/s. The criterion for termination of a reach, marking
the end of the IC (see below), was wrist speed falling below
5 cm/s. The criterion for termination of the grasp closure
and thus, of the CC and of the entire reach-to-grasp move-
ment was when Wnger speed dropped below 5 cm/s. We
measured and analysed nine diVerent dependent variables:
1. Onset Asynchrony (OA): the time between the onset of
the two hands (i.e., the diVerence in when each of the
two hands started moving).
2. Total Task Time (TTT): the total reach-to-grasp time,
deWned as the time taken from the reach onset until the
Fig. 1 Schematic showing layout of the experiment. The right hand
reached to grasp an object of constant width and grasp surface at a Wxed
distance. The left hand reached to grasp objects of three diVeren
t
widths and three diVerent grasp surfaces at one of three diVerent dis-
tances
Exp Brain Res (2008) 184:283–293 287
123
time at which both hands had secured a grasp, that is,
grasp termination.
3. Peak Speed: peak speed reached by the constant (PSc)
and variable (PSv) hand’s wrist.
4. Time to Peak Speed: time to peak speed for the con-
stant (TPSc) and variable (TPSv) hand’s wrist.
5. Normalised Time to Peak Speed: time to peak speed
normalised by Initial Component Duration (see 7
below) for the constant (NTPSc) and variable (NTPSv)
hand’s wrist.
6. Maximum Grip Aperture: the largest distance occur-
ring between the index Wnger and thumb tips during the
reach for the constant (MGAc) and variable (MGAv)
hand.
7. Initial Component Duration: the times from reach onset
(when the hand started moving) to reach termination
(when the wrist stopped moving) for the constant
(ICDc) and variable (ICDv) hand.
8. Initial Component Asynchrony (ICA): the diVerence
between the reach termination times (when each wrist
stopped moving) of the constant and variable hands
(we computed and analysed signed diVerences).
9. Grasp Termination Asynchrony (GTA): the diVerence
between when a grasp was achieved (i.e., when the
Wngers stopped moving) by the constant hand and
when it was achieved by the variable hand (we com-
puted and analysed signed diVerences).
We used the median value for the ten trials for each condi-
tion for each participant within our analyses for each
dependent variable. We selected the median as standard
practice as it is robust to outliers; mean values produced the
same pattern of results. We examined each of these mea-
sures in two ways. First, each was studied for evidence of
deviations from the characteristic form of unimanual reach-
to-grasp movements as determined by the target object
properties: distance, grip area size, and width. That is, we
looked for the appearance of interactions between the hands
causing them to be more synchronous as the reach-to-grasp
evolved through time. Second, each measure was examined
to see if asynchrony was present, especially in the Wnal
stages when the grasp is actually established. On the one
hand, a tendency towards synchrony was expected given
the results of the previous studies of bimanual coordination
described above. On the other hand, we also expected asyn-
chrony near the end of the movements as an unavoidable
consequence of vision being directed towards one hand and
then the other (with necessary corrections then made under
visual guidance).
Results
Control experiment
The data from all ten participants on all 48 trials were
inspected by eye and analysed using Labview routines.
The pattern of hand movements was consistent with the
Wndings reported for the main experiment. The control
experiment was conducted to explore the changes in Wxa-
tion that occur during the experimental task. It was found
that every participant shifted Wxation from one target to
the other after movement onset, on every trial. Fixation
shifted on average at 267 ms into the movement across
the participants (30.6% of movement duration on aver-
age). This result conWrms previous suggestions that large
angular gaze shifts are generated through head move-
ments (Stahl 1999). Moreover, this result shows that task
success rested upon Wxation of both reaching limbs as
they approached the target. The mode of the number of
head movements was one, with the normal pattern con-
sistent with the participant Wxating the constant target
and then shifting gaze to the variable hand’s target. Tri-
als where the participant started with Wxation on the vari-
able target generally occurred when the variable target
was at the close distance. The trials with more than one
head movement generally occurred with the smallest grip
surface.
Main experiment
All data were entered into a three way repeated measures
ANOVA with Greenhouse–Geisser corrections using
Fig. 2 Example of the kinematic proWles found in the experiment
(scaled and oVset) selected at random. The bottom two signals are the
wrist speed proWles. The top two signals show the formation of the grip
aperture. The constant hand’s signals are beneath the variable hand’s
signals (so the dashed and dotted signals belong to the constant hand).
The solid vertical line shows the cessation of the constant hand’s wrist
movement. The dotted vertical line shows when the constant hand se-
cured the object. The dashed vertical line showed when the variable
hand contacted the object
Time (ms)
100 300 500 700 900 1100
288 Exp Brain Res (2008) 184:283–293
123
distance, width and grip surface area as the three indepen-
dent variables. A separate ANOVA was conducted for each
of the eight dependant variables. For variables that entailed
a separate measure for each hand, separate ANOVAs were
conducted on the data for each hand or we did a four factor
ANOVA with hand as a fourth factor. In the majority of sit-
uations there were no reliable interactions in which case we
report only the statistically reliable main eVects. The uncor-
rected degrees of freedom are (2, 14) for main eVects and
(4, 28) for two-way interactions, the corrected values are
provided when we report the F scores.
Onset asynchrony
There was no reliable eVect of distance, width or grip sur-
face area on the onset asynchrony. The hands started their
movement at approximately the same time (average 5 ms
signed onset asynchrony, 14 ms unsigned asynchrony).
We had expected no signiWcant diVerences in this
analysis.
Total task time
The ANOVA yielded a reliable eVect of distance
[F
(1.05,7.35)
= 10.17, P < 0.05] and grip surface area
[F
(1.79,712.53)
=17.09, P < 0.05]. TTT increased (1) as the
average distance increased and (2) as the grip surface area
decreased in size (Fig. 3).
Peak speed, time to peak speed, and normalised
time to peak speed
Distance, width or grip surface area (of the variable hand)
aVected neither the peak speed nor the time to peak speed
of the constant hand. PS of the variable hand was aVected
reliably by distance alone [F
(1.02,7.03)
= 19.91, P <0.05] as
was the time to peak speed [F
(1.06,7.42)
= 19.03, P < 0.05].
These results can be seen in Fig. 4. There were no signiW-
cant eVects of distance, width or grip surface area on nor-
malised time to peak speed for either the constant or
variable hand. Peak speed occurred at about 40% of the
total reach duration (mean = 44.2%) as typically found in
prehension studies.
The lack of interaction between the hands during the Wrst
part of the reach movements has been reported in previous
studies on bimanual prehension that have analysed initial
reaction times, in particular (Diedrichsen et al. 2001;
Kunde and Weigelt 2005; see Ivry et al. 2004 for review
and discussion). This lack of interaction means that at least
up to the moment of peak speed, reaches to equivalent
objects at the same distances were approximately synchro-
nous while reaches to objects at diVerent distances were
asynchronous.
Maximum grip aperture
The maximum grip aperture of the variable hand was inXu-
enced by object width [F
(1.05,7.34)
= 52.47, P <0.05] and
grip surface area [F
(1.31,9.13)
=22.37, P < 0.05]. Notably,
MGA of the constant hand was also inXuenced by (variable
hand) object width [F
(1.18,8.23)
= 13.45, P<0.05]. This was
the Wrst sign of any interaction between the hands and,
given that MGA occurred at about 70% of the reach dura-
tion, it took place during the deceleration phase of the
reaches. These results can be seen in Fig. 5.
Initial component duration
We performed a four factor repeated measures ANOVA on
the time at which the wrist stopped moving (at the end of
the IC) with hand, distance, width, and grip surface area as
factors. Main eVects were obtained for hand [F
(1,7)
= 6.37,
P<0.05], distance [F
(1.08,7.54)
= 14.57, P<0.01], and grip
Fig. 3 Total task time (ms) was deWned as the time between the Wrst
hand starting to move forward and the time at which both hands had
secured a grasp on the target objects. The Wgure (a) shows that total
task time increases as distance (cm) increases. The Wgure (b) shows
that total task time increases as grasp surface size (cm) decreases. The
lines shown are least-squares linear regressions
Distance (cm)
45
Total task time (ms)Total task time (ms)
660
680
700
720
740
760
Grasp surface size (cm)
695
700
705
710
715
720
725
15 20 25 30
35 40
3.5
0.5 1.0 1.5 2.0 2.5 3.0
A
B
Exp Brain Res (2008) 184:283–293 289
123
surface area [F
(1.08,7.53)
= 7.73, P<0.05]. The constant
hand was faster than the variable hand (constant = 651 ms;
variable = 680 ms). Smaller grip surface area yielded
slower reaches as shown in Fig. 6b. Distance and width
both exhibited interactions with hand: distance by hand
[F
(1.22,8.52)
= 7.98, P<0.05], and width by hand
[F
(1.22,8.54)
= 5.25, P<0.05]. Given these interactions, we
performed a separate three-factor ANOVA on the data for
each hand. The time at which the wrist stopped moving was
aVected reliably by distance (of the variable hand target) for
both the constant [F
(1.75,12.26)
= 14.51, P<0.05] and vari-
able hand [F
(1.09,7.62)
= 11.99, P<0.05]. Likewise, the time
at which the hand stopped moving was aVected reliably by
grip surface area for both the constant [F
(1.09,7.64)
= 4.15,
P<0.05] and variable hand [F
(1.15,8.06)
= 12.37, P<0.05].
These results can be seen in Fig. 6. The eVect of distance
and grasp surface size on the timing of the constant hand
was the second result showing interaction between the
hands.
The changes in constant hand timing were not nearly
large enough to yield synchronous movements as can be
seen in Fig. 6. The asynchrony of timing was shown by the
statistically signiWcant interaction of hand with both dis-
tance and grip surface area in the four-factor ANOVA. The
main eVect found for the hand in the four-factor ANOVA
revealed that when all else was equal, the constant hand
would arrive Wrst. In the context of the relative phase mea-
sure used in rhythmic bimanual coordination studies, this
t80 ms asynchrony in movements of t640 ms duration
represented a phase diVerence of about 20°.
Initial component asynchrony
We next tested the asynchrony at the end of the IC directly.
We calculated the diVerence between when the constant
and variable wrist stopped moving (the end of the IC) in
each trial. The temporal diVerence between the two hands
was aVected reliably by distance [F
(1.05,7.34)
= 18.22,
Fig. 4 Peak tangential speed (mm/s) plotted against distance in cm
(upper, a) and time to peak tangential speed (ms) plotted against dis-
tance in cm (lower, b). The peak speed and the time to peak speed o
f
the variable hand were aVected by average distance. The target of the
variable hand did not aVect the peak speed of the constant hand. The
lines shown are least-squares linear regressions
Distance (cm)
45
800
900
1000
1100
1200
1300
Variable
Constant
Distance (cm)
260
270
280
290
300
310
320
Variable
Constant
Peak speed (mm/s)
Time to peak speed (ms)
15 20 25 30
35
40
45
15 20 25 30
35
40
A
B
Fig. 5 Maximum grip aperture (cm) of both the constant and variable
hand plotted against object width in cm (upper, a) and against grip sur-
face in cm (lower, b) for the variable hand. The inXuence of the vari-
able hand target’s width on the maximum grip aperture of the constant
hand was the Wrst sign of any interaction between the hands. The lines
shown are least-squares linear regressions
Object width (cm)
8
Maximum grip aperture (cm)
Maximum grip aperture (cm)
8.5
9.0
9.5
10.0
10.5
11.0
11.5
Variable
Constant
Grasp surface size (cm)
8.5
9.0
9.5
10.0
10.5
11.0
11.5
Variable
Constant
40
2
3
456
7
3.5
0.5
1.0
1.5
2.0
2.5 3.0
290 Exp Brain Res (2008) 184:283–293
123
P<0.05] and object width [F
(1.62,11.36)
= 5.56, P<0.05] as
shown in Fig. 7. (These were the factors that interacted
with hand in the previous analysis.) Figure 7 shows that the
order of the hands reversed as predicted (with the nearer
target reached Wrst and the further target reached last). An
examination of the frequency histograms of these asynchro-
nies at each distance revealed that for the 20 and 40 cm tar-
gets (i.e., diVerent target distances for the two hands), about
90% of the data were in the direction represented by the
means shown in Fig. 7 and the coeYcients of variation (i.e.,
SD/mean) were about 1.0. In contrast, the mean asynchrony
for the 30 cm distance (i.e., the equal distance condition)
was small (t¡20 ms). Examination of the frequency histo-
gram revealed that the data were distributed about both pos-
itive and negative asynchronies signiWcantly diVerent from
zero, but with distributions that overlapped at zero. We
computed mean (and SD) asynchronies for the positive
(variable hand Wrst) and negative (constant hand Wrst) dis-
tributions as object width varied from small to large. See
Table 1. In all cases, the mean absolute value of asyn-
chrony, whether positive or negative, was about 76 ms with
a coeYcient of variation of about 86% (i.e. SDs were about
65 ms). Both positive and negative asynchronies varied
with object width as shown by two-tailed t-tests,
F(98) = 756.4, P<0.01, and F(136) = 1458.4, P<0.01,
respectively. Thus, in the equal target distance condition,
which hand arrived Wrst appeared to vary randomly, but in
about half of the trials the constant hand arrived Wrst by
about 76 ms on average, and in the other half, the variable
hand arrived Wrst by about the same amount of time.
Grasp termination asynchrony
We calculated the temporal diVerences between when the
constant hand and variable hand achieved a Wnal grasp for
each individual trial and analysed these data. A reliable
interaction was found between the GTA and both reach dis-
tance and grip surface area [F
(2.81,19.64)
= 4.03, P<0.05].
Fig. 6 The initial component duration (ms) for the constant and vari-
able hand plotted against distance in cm (upper, a) and against grasp
surface size in cm (lower, b). The eVect of distance and grasp surface
size on the timing of the constant hand was the second result showing
interaction between the hands. The lines shown are least-squares linear
regressions
Distance (cm)
45
620
640
660
680
700
720
740
Variable
Constant
Grasp surface size (cm)
630
640
650
660
670
680
690
700
710
Variable
Constant
Initial component duration (ms)
Initial component duration (ms)
15 20
25
30 35
40
3.5
0.5 1.0
1.5
2.0 2.5
3.0
Fig. 7 The signed temporal diVerence (ms) between when the con-
stant and variable hand stopped moving plotted against distance in cm
in the upper plot (a) and against object width (cm) in the lower plot (b).
The lines shown are least-squares linear regressions. It can be seen that
the order of the hands reversed according to target distance (with the
nearer target reached Wrst by the variable hand and the further targe
t
reached last)
Distance (cm)
Reach termination asynchrony (ms)
Reach termination aynschrony (ms)
-80
-60
-40
-20
0
20
40
60
Variable hand
finishes before
constant
Object width (cm)
-35
-30
-25
-20
-15
-10
-5
0
Variable hand
finishes after
constant
A
B
45
15 20 25 30
35 40
8
2345
67
Exp Brain Res (2008) 184:283–293 291
123
There was also a main eVect of object width on GTA
[F
(1.73,12.71)
= 8.15, P<0.05]. The results shown in Fig. 8
are essentially the same as in Fig. 7, that is, the timing at the
end of the IC was preserved through to the end of the entire
reach-to-grasp movement.
Discussion
We investigated bimanual prehension to separate target
objects that varied in distance and size. We did not explic-
itly instruct the participants to move their hands together
but previous research (e.g. Diedrichsen et al. 2001; Kelso
et al. 1979a,b; Kunde and Weigelt 2005) led us to predict
that: (1) the participants would start to move the hands at
the same time and (2) the hands would move in reasonable
temporal proximity (i.e. the movement of the two hands
would occur in a common time window). We observed this
expected pattern in all participants. However, we also
expected target distance and size to aVect the reach-to-
grasp in the usual ways, which is to say that reaches should
take longer to targets that are more distant or smaller. As
shown in Fig. 3, these expectations were met. Furthermore,
the peak speeds and the times for acceleration to peak speed
for each hand were strictly a function of the targets
approached by each hand. Thus, the peak speed and the
time to peak speed were constant for the constant hand and
variable for the variable hand.
The central question was how would the two hands
coordinate in the terminal phase of the bimanual reaches?
Would the reaches-to-grasp be synchronous or asynchro-
nous? On the one hand, previous studies of reaching
(without grasping) have found relative synchrony through-
out (e.g. Kelso 1979a,b). On the other hand, the task
requirements for visual guidance led us to predict that asyn-
chrony would be observed in the movements (e.g. Fisk and
Goodale 1988; Winges et al. 2003). In fact, we did see a
tendency for synchronisation between the hands during the
deceleration portion of the reaches. The movement of one
hand started to inXuence the movement of the other hand so
the variable hand’s target began to inXuence the kinematics
recorded for the constant hand. Variations in the width of
the variable hand target aVected the maximum grasp aper-
ture of the constant hand. Furthermore, the deceleration
phase of the constant hand was aVected by the variable
hand target as indexed by the mean times at the end of the
reaches (i.e., the Initial Component Durations). The arrival
of the constant right hand lengthened as variable hand’s tar-
get became further and its surface became smaller. The
inXuence of grip surface area on maximum grasp aperture
(MGA) was probably mediated by this eVect. The maxi-
mum grip aperture of the variable hand altered in response
to grasp surface size as well as to object width. A small
grasp surface resulted in a decreased MGA whereas a large
grasp surface resulted in an increased MGA. The changes
in grasp aperture can be explained by the shorter decelera-
tion phase found in both hands when moving to a larger
grasp surface (a larger grasp surface decreases accuracy
demands allowing the system to reduce feedback correction
and thus produce faster movements). It is known that
Table 1 Mean (and SD) asynchronies (in ms) at the end of the IC for
the 30 cm variable hand target (i.e. equal target distances for the two
hands) for constant hand Wrst (negative) and variable hand Wrst (posi-
tive) trials by object width
Object width
Small Medium Large
Mean ¡84.71 ¡85.57 ¡80.95
SD 71.51 77.75 65.35
Mean 77.52 63.11 60.74
SD 76.89 46.41 58.66
Fig. 8 The upper plot (a) shows the signed temporal diVerences (ms)
between when the constant hand and variable hand achieved a stable
grasp plotted against distance (cm) for the three diVerent grasp sur-
faces. The lower plot (b) shows the temporal diVerences (ms) between
when the constant hand and variable hand achieved a stable grasp plot-
ted against object width (cm). The lines shown are least-squares linear
regressions. The pattern of results is similar to that shown in Fig. 6
indicating that the timing at the end of the IC was preserved through to
the end of the entire reach-to-grasp movement
-25
-20
-15
-10
-5
0
5
10
Variable hand
finishes before
constant
constant
B
A
Distance (cm)
Grasp termination asynchrony (ms)
Grasp termination asynchrony (ms)
-100
-80
-60
-40
-20
0
20
40
60
80
Narrow
Medium
Wide
Variable hand
finishes before
45
15 20 25 30
35 40
Object width (cm)
8
2
3
45
67
292 Exp Brain Res (2008) 184:283–293
123
alterations in movement time produce reliable changes in
grasp aperture formation (Wing et al. 1986; Smeets and
Brenner 1999; Mon-Williams and Tresilian 2001; Loftus
et al. 2004). Nonetheless, these interactions were not nearly
enough to yield synchrony between the hands when they
reached towards unequal target objects at diVerent dis-
tances. Moreover, when the hands reached towards equiva-
lent targets at equal distances, their arrival was
asynchronous. There was some tendency for the constant
hand to arrive sooner, but which hand arrived Wrst was
largely random. When the targets were diVerent distances,
then the order of arrival was determined by the relative
proximity of the targets as predicted (with the closer target
reached Wrst). It should be noted that the precise pattern we
observed might have altered if the non-preferred hand had
been reaching for the constant target (with the asynchrony
and interaction of the hands being either larger or smaller
during a particular movement phase). Nevertheless, this
study shows the generic constraints that aVect bimanual
reach-to-grasp behaviour.
In conclusion, diVerences in the distances of targets did
yield asynchronies in reaching as expected from normal uni-
manual behaviour. These asynchronies were used to deter-
mine the ordering of the hands in closing on the targets.
Increasing the diYculty of the target increased the magni-
tude of the asynchrony. Furthermore, when the targets dis-
tances were equal, the hands still exhibited asynchronies in
arrival times, but now the ordering of the hands was largely
random. The asynchronies that accrued during the IC were
preserved through the CC. The parsimonious explanation is
that the asynchronies were driven by the need for visual
information to guide each hand in turn. Riek et al.’s (2003)
eye movement study suggests that these asymmetries reXect
looking behaviours and the time taken to Wxate and guide
Wrst one hand and then the other. Consistent with this con-
clusion, the control experiment showed that every partici-
pant shifted Wxation at least once (sometimes more when
task diYculty increased) on every trial. The normal Wxation
shift pattern was consistent with the participant Wxating the
constant target and then shifting gaze to the variable hand’s
target. The strategy of Wxating on the variable target last by
the participants (as evidenced by the control experiment)
coincides well with the Wnding of increasing asynchrony
with increasing target diYculty. The eVect of this did not
appear until the stage at which visual feedback was actually
being used to guide the reach (i.e. during deceleration).
Until the deceleration phase, the two reaches did not interact
and each followed a trajectory determined stably by the task
and the distance of the given target.
In summary, this article has shown the lawful relation-
ships that are present in bimanual prehension movements.
The experimental task often produced a qualitatively diVer-
ent prehension pattern than that sometimes described in the
past studies. This result shows the Xexibility of the prehen-
sion pattern in response to task demands. We set out to
determine whether the bias for synchronicity would deter-
mine the coordinative timing of bimanual prehension
movements to separate objects. We found that the behav-
iour was biased towards synchrony but the need for visual
information also inXuenced the coordinative timing, pro-
ducing asynchrony in reliable ways.
Acknowledgment We are grateful to Jennifer Leaper (Charles) and
Silke Leifheit for conducting the control experiment and analysing the
data. A grant to Mark Mon-Williams from Action Medical Research
and Scottish Enterprise helped support this research work. The produc-
tion of the manuscript was facilitated by an award from the Royal
Society of Edinburgh/Lloyds TSB to Bingham and Mon-Williams.
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